A simple field method to measure the hydrodynamic properties of soil surface crust

The hydraulic resistance of the surface crust was determined by a combination of two infiltrometric techniques: first, a surface measurement of steady-state infiltration rate is conducted by a mini-disk tension infiltrometer (MDI); then, the surface crust is removed, its thickness is measured, and a ponded infiltration test is performed at the same site. The Beerkan Estimation of Soil Transfer parameters (BEST) method is applied to estimate the hydraulic properties of the underlying soil provided the particle-size distribution and the bulk density are known. Under the assumption of a unit gradient of hydraulic head below the soil crust, the pressure head at the interface crust-soil is derived. Finally, the hydraulic conductivity of the crust is calculated from the steady-state water flow measured by the MDI and the Darcy law. The method was tested in a sandy loam and a clay soil. In the sandy loam soil, a 2-3 mm thick slaking crust was visually observed, but no increased surface hydraulic resistance was detected in 10 out of 11 cases. In the clay soil, a 5-7 mm thick crust was formed by gradual coalescence of the plastic, wet aggregates by rainfall compaction. In 10 out of 15 tests, the steady-state infiltration rate with the crust was lower than the underlying soil saturated hydraulic conductivity, denoting an increased hydraulic resistance of the surface crust. For the clay soil, the mean value of the hydraulic resistance was practically independent of the crust thickness and varied between 78 and 81 min.